The present invention relates to an intake system of an internal combustion engine.
For improving a specific fuel consumption, a method of using a lean mixture is known. However, since the propagating speed of flame of a lean mixture is inherently low and, thus, the burning velocity is low, it is difficult to obtain a stable combustion. Particularly in the case where a so-called super lean mixture, having an air-fuel ratio which is larger than 20:1, is used, since the burning velocity becomes even lower, it is more difficult to obtain a stable combustion. For increasing the burning velocity, a method of creating a swirl motion in the combustion chamber is known. However, it has been proven that, if such a swirl motion is too strong or too weak, it is difficult to obtain a stable combustion, and that an optimum strength of the swirl motion is present in accordance with kinds of engine types.